1. Field of the Invention
The invention relates to photographic apparatus including an instant of self-developing type camera and a film assemblage which interrelate with each other to control the thickness and/or shape of a layer of processing liquid to be spread between layers of an exposed film unit.
2. Description of the Prior Art
The invention is directed to certain improvements in the relationship between photographic film assemblages of the self-developing type and the cameras (or camera backs) with which they are adapted to be used. More specifically, the invention relates to a relationship between the two which allows for a greater spacial juxtapositioning between processing liquid spread control features on a film cassette and the liquid spreading and control structures in a camera.
As pointed up in U.S. Pat. No. 3,799,770, an important step in the processing of an exposed instant or self-developing type film unit is the spreading of a processing liquid between predetermined layers of the film unit to initiate the formation of a visible image therein via a diffusion transfer process. In order to obtain an optimum quality positive print, it is desirable that the processing liquid be spread such that it covers the entire photoexposed area of the film unit in a thin layer of predetermined uniform thickness.
A typical film unit includes a photosensitive element, an image-receiving element which may be superposed on the photosensitive element subsequent to exposure or may be transparent and predisposed on the photosensitive element such that exposure may be made through the image-receiving element, and a rupturable pod or container of processing liquid located at one end of the two elements.
In a typical self-developing photographic system, the film units are arranged in stacked relation within a film cassette which is adapted to be inserted into the receiving chamber of an appropriate camera to locate an endmost film unit in the stack in position for exposure.
Subsequent to exposure, the endmost film unit is extracted from the film cassette and is advanced, pod end first, between a pair of pressure-applying members mounted within the camera. The pressure-applying members exert a compressive force on the pod causing it to rupture and discharge the liquid between predetermined layers of the film unit, e.g., between the exposed photosensitive element and the superposed image-receiving element. Continued advancement of the film unit between the pressure-applying members results in the liquid being advanced along a liquid wave front toward the trailing end of the film unit such that it is progressively distributed over the photoexposed area of the film unit.
The uniformity of the liquid layer is, to a large degree, determined by the initial shape of the liquid wave front. In order to uniformly spread the processing composition over a substantially rectangular or square photoexposed area it is preferable that the wave front be disposed in a substantially straight line which extends outwardly to the lateral margins of the area and is oriented in a direction that is normal to the direction of film advancement between the pressure-applying members.
There are several factors which effect the initial shape of the wave front. One is a design of the pod and its rupture characteristics. Another relates to the viscosity and amount of liquid enclosed by the pod. The wave front shape is also influenced by the velocity at which the film unit is advanced through the pressure-applying members, the amount of compressive pressure exerted on the film unit, and the resistance to liquid flow at the interfaces between the liquid and the superposed film unit elements.
One of the most commonly observed spread shapes is a tongue shape wherein the wave front progresses more rapidly at the central portion of the photoexposed or image-forming area than out at the lateral margins. This condition may be caused by an uneven distribution of liquid upon initial discharge from the pod, i.e., more liquid being concentrated at the center of the film unit than out at its edges. In the subsequent spreading of the tongue-shaped wave front, it is possible that the corners of the image-forming area at the trailing end of the film unit will be coated with a layer of liquid of reduced depth or thickness, thus, possibly adversely affecting the film unit's sensitometry or not be coated at all.
In an instant type film unit of the "integral type", as shown in U.S. Pat. No. 4,693,963, the film unit is configured as including first and second superposed sheets, at least one of which comprises photosensitive constituents, whose lateral edges are permanently secured to each other by longitudinally extending rails. Because these sheets are bound at their lateral edges, the sheets tend to separate more in the center of the film unit than out at the lateral margins in response to the processing liquid being spread therebetween. Thus, there is more resistance to the flow of the processing liquid at the edges of the film unit vis-a-vis its center section. Upon initial discharge of the liquid from its pod, it assumes a rearwardly extending tongue shape rather than proceeding toward the trailing end of the film unit along a uniform wave front.
One method employed to compensate for a tongue-shaped wave front has been to provide excess liquid in the pod. Another method has been to equip the camera with spread control devices which serve to modify the shape of the liquid wave front during spreading.
The spread control devices are designed to apply a second compressive force to the central portion of the film unit in the path of the mass of liquid discharged from the pod by the pressure-applying members. This serves to retard the central portion of the wave front and cause a flow of liquid in a direction transverse to the direction of film advancement. In this manner, the wave front is modified such that it is substantially straight and is oriented in a direction substantially normal to the parallel sides of the rectangular or square image-forming area.
For examples of cameras which include processing liquid spread control means reference should be had to U.S. Pat. Nos. 3,241,468, 3,777,647 and 3,810,211. For examples of film assemblages which include film cassettes having processing liquid spread control features reference should be had to U.S. Pat. Nos. 3,779,770, 3,832,731, 4,104,669 and 4,226,519.
A major drawback with systems of the type shown and described in the patents listed above is that the control features located on the film cassette are formed on interior surfaces thereof and thus must be spaced relatively close to the pressure-applying means or spread rollers of the camera in order to be effective. This severely restricts the options available to a camera designer in the placement of the spread rollers relative to the film cassette. Further, if spreading of the processing liquid takes place substantially at the location where the film unit emerges from its cassette, then it is generally advisable that the film unit be maintained in a planar condition until processing of the image has been substantially completed. This is so as not to subject that layer of processing liquid to any external forces, such as may be caused by bending the film unit during its transport to a storage chamber or to the exterior of the camera.